1. Field of the Invention
The present invention relates generally to the system and method for controlling intake air flow for an engine and particularly to a system and method wherein the speed and response of intake manifold pressure to depression of an accelerator pedal or operation of such an acceleration mechanism varies according to engine operating conditions and transmission gear position so that the responsiveness of the engine can be improved together with drivability without unfavorable vibrations.
2. Description of the Prior Art
Most gasoline engines used nowadays in vehicles have throttle valves mounted in intake manifolds and mechanically linked to acceleration mechanism such as accelerator pedals. Therefore, the vehicle driver can control engine output directly through depression of the accelerator pedals.
FIG. 1 shows an example of a conventional intake air flow control system.
In FIG. 2, numeral 1 denotes an airflow meter for measuring intake air quantity, numeral 2 denotes a throttle valve, numeral 3 denotes an intake manifold, numeral 4 denotes an intake air valve or intake air valves of an engine cylinder 7, numeral 5 denotes an exhaust valve or exhaust valves, numeral 6 denotes a piston of the engine cylinder 7, and numeral 8 denotes an accelerator pedal.
Since the accelerator pedal 8 is mechanically linked to the throttle valve 2, as represented by the dotted line in FIG. 1, the angle through which the accelerator pedal 8 is depressed directly corresponds to the angle through which the throttle valve 2 is opened.
FIG. 2 illustrates the response of the system shown in FIG. 1 with regard to intake air flow in response to a rapid change in the opening angle of the throttle valve.
In FIG. 2, symbol A1 denotes the flow cross-section past the throttle valve 2 and symbol P1 denotes the equilibrium pressure in the intake manifold 3 (intake passage negative pressure) when the engine is operating at the above-described flow cross-section A1. The symbol A2 denotes a subsequent flow cross-section resulting when the throttle valve 2 is abruptly actuated to a new opening angle and symbol P2 denotes the subsequent equilibrium pressure in the intake manifold 3 when the engine is operating at the above-described flow cross-section A2.
If the volume of the engine cylinder 7 is Vh, the volume of the intake manifold 3 is V, and engine revolution speed is N, the pressure in the intake manifold 3 changes from P1 to P2 with an exponential time constant .tau. as expressed below in the case where the opening angle of the throttle valve is changed in a stepwise manner from A1 to A2.
.tau.=V/(C1.multidot.A2+C2.multidot.Vh.multidot.N), where C1 and C2 are constants.
As appreciated from the expression, the time constant .tau. is proportional to the volume of the intake manifold V and inversely proportional to the sum of the flow cross-section A for intake air flow and the intake air flow rate into the engine through the intake manifold 3, i.e., Vh.times.N.
In addition, the intake air quantity Q drawn into each engine cylinder 7 in each engine revolution is given by multiplying the intake manifold pressure P by the charging efficiency .eta. determined by the engine speed or torque (Q=.eta..multidot.P).
Hence, assuming an amount of fuel accurately corresponding to the intake air quantity is supplied to the engine through a carburetor or fuel injection valve, the response of the output torque of the engine which is proportional to the charging efficiency .eta. substantially matches that of the intake manifold pressure P.
As appreciated from the above-mentioned expression, the time constant relates the intake manifold pressure P inversely to the flow cross-section (opening angle of the throttle valve) assuming the engine revolution speed N is constant. That is to say, the response is slower in a low-load range in which the throttle valve 2 is nearly closed and, on the other hand, is faster in a high-load range in which the throttle valve 2 is almost fully opened.
Taking the responsiveness of the engine as a whole into consideration, it is preferable to achieve the fastest possible intake manifold pressure response. On the other hand, it is preferable to have a slower response in order to prevent vehicle vibrations which may occur when the engine decelerates or accelerates (so-called jerk). This is especially true when the transmission is in a lower gear position (first gear or second gear).
However, in the conventional system shown in FIG. 1, it is impossible to freely control the degree of responsiveness described above since the throttle valve is mechanically linked to the accelerator pedal. In addition, although Japanese Patent Laid Open Application No. Sho. 50-43626 and Japanese Patent Laid Open Application No. Sho. 56-107925 disclose a method wherein the angle through which the accelerator pedal is depressed is electrically detected and the opening angle of the throttle valve is controlled on the basis of the position of the accelerator pedal and other engine operating conditions, these methods do not control the responsiveness of the intake manifold pressure described above.